That's fucking hilarious!! She reads the joke "Once you go black... you're a single mom," trying to be all SJW "Isn't that horrible?!" But Lex is all turned on by it, thinkin' she's just talking dirty to entice him. "Ooh yeah, baby! I'm gonna knock you up and take right the fuck off- Oh, wait. I'm supposed to be offended by that. Oops." Like most jokes, it's funny because it's so very fucking true.